The surface energy of UO2 as determined by hertzian indentation

Hertzian indentation fracture has been evaluated as a means of determining the fracture surface energy γ of brittle solids and has been applied experimentally to the measurement of the fracture surface energy of UO₂. The method was found to be both sensitive and experimentally simple. Though the method requires further development to give reliable absolute values of γ, it seems to be capable of revealing the relative values of different samples. On the basis of a simplified approximation of the stress intensity factor for the Hertzian ring crack, the fracture surface energy of UO₂ was found to be (1.8 ± 0.3) J/m² which compares favorably with the value of (2.5 ± 0.2) J/m² determined for ThO₂ with the same method. For UO_{2 + x}, γ increased with x.     

Vaporization study on UO2 and (U1−yNby)O2+x by mass-spectrometric method

The vapor pressures over UO_2.000 and (U_1?yNb_y)O_2+x (y = 0.01, 0.05, x = 0.000–0.022) were measured by the mass-spectrometric method in the temperature range 2025–2343 K. The main gas species over UO_2.000 were observed to be UO₃(g) and UO₂(g) and those over (U_1?yNb_y)O_2+x were NbO₂(g), NbO(g), UO₃(g) and UO₂(g). The partial vapor pressures of almost all gas species over (U_1?yNb_y)O_2+x increased with increasing O/M (M = U + Nb) ratio. With increasing Nb content in (U_1?yNb_y)O_2.000, the partial vapor pressures of UO₂(g) and UO₃(g) decreased and those of NbO(g) and NbO₂(g) increased. The congruently vaporizing composition in the (U_1?yNb_y)O_2+x phase was estimated to be ($\text{U}{}_{\mathrm{0.985\pm 0.005}}\text{Nb}{}_{\mathrm{0.015\pm 0.005}}\text{)O}{}_{2.000}$ from the compositional dependence of the total vapor pressures. The partial molar enthalpy and entropy of oxygen of (U_1?yNb_y)O_2+x calculated from the partial pressures of gaseous species NbO₂(g) and NbO(g) were in fairly good agreement with those previously obtained by the present authors with a thermobalance.     

The creep of UO2 fuel doped with Nb2O5

The creep of UO₂ containing small additions of Nb₂O₅ has been investigated in the stress range 0.5–90 MN/m² at temperatures between 1422 and 1573 K. The functional dependence of the creep rate of five dopant concentrations up to 0.8 mol% Nb₂O₅ has been examined and it was established that in all the materials the secondary creep rate could be represented by the equation /.εkT = Aσⁿexp(?Q/RT), where /.ε is the steady state creep rate per hour, Q the activation energy and A and n are constants for each material. It was observed that Nb₂O₅ additions can cause a dramatic increase in the steady state creep rate as long as the niobium ion is maintained in the Nb⁵⁺ valence state. Material containing 0.4 mol% Nb₂O₅ creeps three orders of magnitude faster than the pure material.Analysis of the results in terms of grain size compensated viscosity suggest that, like “pure” UO₂, the creep rate of Nb₂O₅ doped fuel is diffusion-controlled and proportional to the reciprocal square of the grain size. A model is developed which suggests that the increase in creep rate results from suppression of the U⁵⁺ ion concentration by the addition of Mb⁵⁺ ions, which modifies the crystal defect structure and hence the uranium ion diffusion coefficient.     

Investigation of ThO2 and (U, Th)O2

The effect of the properties of ThO₂ and (U, Th)O₂ powders, prepared with different technological regimes, on the properties of the finished items is investigated. The work includes detailed investigations of ThO₂ and (U, Th)O₂ powders (x-ray phase analysis, electron-microscope investigation) and sintered fuel pellets (determination of density, study of microstructure, thermophysical investigations). The temperature dependences of the crystal lattice parameters and the sizes of the crystallites in ThO₂ and (U, Th)O₂ powders with different UO₂:ThO₂ ratio are obtained. The temperature dependences of the thermal conductivity of sintered ThO₂ and (U, Th)O₂ pellets with different UO₂:ThO₂ ratio are studied.     

Characterization and densification studies on ThO2-UO2 pellets derived from ThO2 and U3O8 powders

ThO₂ containing around 2-3% ²³³UO₂ is the proposed fuel for the forthcoming Indian Advanced Heavy Water Reactor (AHWR). This fuel is prepared by powder metallurgy technique using ThO₂ and U₃O₈ powders as the starting material. The densification behaviour of the fuel was evaluated using a high temperature dilatometer in four different atmospheres Ar, Ar-8%H₂, CO₂ and air. Air was found to be the best medium for sintering among them. For Ar and Ar-8%H₂ atmospheres, the former gave a slightly higher densification. Thermogravimetric studies carried out on ThO₂-2%U₃O₈ granules in air showed a continuous decrease in weight up to 1500 °C. The effectiveness of U₃O₈ in enhancing the sintering of ThO₂ has been established.     

Kinetics of the carbothermic reduction of a ThO2 + UO2 + C mixture to prepare (Th,U)C

Carbothermic reduction of mechanically mixed ThO₂ + UO₂2 + C compact to (Th, U)C has been studied in the temperature range between 1743 and 2043 K with emphasis on reaction kinetics. The rate-limiting step of this reaction was attributed to the diffusion of CO gas in the outermost layer of the reaction products. By X-ray diffraction, ThO₂ and UO₂ were found to react with graphite respectively to produce two nearly separate dicarbide phases, both of which then reacted with residual ThO₂ to form a monocarbide phase. An apparent activation energy of about 320 kJ mol⁻¹ was obtained for this carbothermic reduction. Such a high activation energy can be explained by the CO gas diffusion through micropores in that product layer, by taking into account the standard enthalpy changes of the related reactions to produce CO gas.     

Phasengleichgewichte in den systemen UO2-UO2,67-ThO2 und UO2 + x-NPO2

Solid-state chemical investigations have established that in the compositional range UO₂-UO_2.67-ThO₃ of the U-Th-O ternary system, the following single-phase domains exist: U₃O₈, which does not dissolve any ThO₂ in the solid state; an ordered M₄O₉ phase on the section between U₄O₉ and U₂Th₂O₉, below ≈ 1150 °C; and a phase with fluorite structure which occupies a large part of the system and which at 1250 °C is bounded by the compositions UO₂-UO_2.25 (U_0.43, ThO_0.57)O_0.25-ThO₃. The maximum O/M ratio of the “fluorite” phase is O:(U + Th) = 2.25. The highest oxidation valency of uranium is 5.30; this value falls as more thorium oxide is incorporated in the (U.Th)O_{2 + x} “fluorite” phase.     

Dissolution of ThO2Based Oxides in Nitric Acid Solutions at Elevated Temperatures

Highly-dense spherical particles of thorium-based oxides, ThO₂ and (Th, U)O₂, prepared by the sol-gel method were subjected to dissolution with nitric acid containing 0–0.05 mol/l NaF at high temperatures above 120°C. The dissolution rate depended upon temperature, fluoride concentration and UO₂ content. High-temperature in the range of 120–200°C enhanced the dissolution of the ThO₂-based fuels. At low temperatures and/or low U0₂ concentrations, insoluble tetrafluoride precipitates were formed on the particle surfaces and they resulted in the decrease of the dissolution rates. In the present study, the apparent activation energies for the high-temperature dissolution were obtained.     

Low cost, proliferation resistant, uranium–thorium dioxide fuels for light water reactors

Our objective is to develop a fuel for the existing light water reactors (LWRs) that, (a) is less expensive to fabricate than the current uranium-dioxide (UO₂) fuel; (b) allows longer refueling cycles and higher sustainable plant capacity factors; (c) is very resistant to nuclear weapon-material proliferation; (d) results in a more stable and insoluble waste form; and (e) generates less high level waste. This paper presents the results of our initial investigation of a LWR fuel consisting of mixed thorium dioxide and uranium dioxide (ThO₂–UO₂). Our calculations using the SCALE 4.4 and MOCUP code systems indicate that the mixed ThO₂–UO₂ fuel, with about 6 wt.% of the total heavy metal U-235, could be burned to 72 MW day kg⁻¹ (megawatt thermal days per kilogram) using 30 wt.% UO₂ and the balance ThO₂. The ThO₂–UO₂ cores can also be burned to about 87 MW day kg⁻¹ using 35 wt.% UO₂ and 65% ThO₂with an initial enrichment of about 7 wt.% of the total heavy metal fissile material. Economic analyses indicate that the ThO₂–UO₂ fuel will require less separative work and less total heavy metal (thorium and uranium) feedstock. At reasonable future costs for raw materials and separative work, the cost of the ThO₂–UO₂ fuel is about 9% less than uranium fuel burned to 72 MW day kg⁻¹. Because ThO₂–UO₂ fuel will operate somewhat cooler, and retain within the fuel more of the fission products, especially the gasses, ThO₂–UO₂ fuel can probably be operated successfully to higher burnups than UO₂ fuel. This will allow for longer refueling cycles and better plant capacity factors. The uranium in our calculations remained below 20 wt.% total fissile fraction throughout the cycle, making it unusable for weapons. Total plutonium production per MW day was a factor of 3.2 less in the ThO₂–UO₂ fuel than in the conventional UO₂ fuel burned to 45 MW day kg⁻¹. Pu-239 production per MW day was a factor of about 4 less in the ThO₂–UO₂ fuel than in the conventional fuel. The plutonium produced was high in Pu-238, leading to a decay heat about three times greater than that from plutonium derived from conventional fuel burned to 45 MW day kg⁻¹ and 20 times greater than weapons grade plutonium. This will make fabrication of a weapon more difficult. Spontaneous neutron production from the plutonium in the ThO₂–UO₂ fuel was about 50% greater than that from conventional fuel and ten times greater than that from weapons grade plutonium. High spontaneous neutron production drastically limits the probable yield of a crude weapon. Because ThO₂ is the highest oxide of thorium while UO₂ can be oxidized further to U₃O₈ or UO₃, ThO₂–UO₂ fuel appears to be a superior waste form if the spent fuel is to be exposed ever to air or oxygenated water. And, finally, use of higher burnup fuel will result in proportionally fewer spent fuel bundles to handle, store, ship, and permanently dispose of.     

Corrosion of UO2 and ThO2: A quantum-mechanical investigation

The addition of Th to U-based fuels increases resistance to corrosion due to differences in redox-chemistry and electronic properties between UO₂ and ThO₂. Quantum-mechanical techniques were used to calculate surface energy trends for ThO₂, resulting in (1 1 1) < (1 1 0) < (1 0 0). Adsorption energy trends were calculated for water and oxygen on the stable (1 1 1) surface of UO₂ and ThO₂, and the effect of model set-up on these trends was evaluated. Molecular water is more stable than dissociated water on both binary oxides. Oxidation rates for atomic oxygen interacting with defect-free UO₂(1 1 1) were calculated to be extremely slow if no water is present, but nearly instantaneous if water is present. The semi-conducting nature of UO₂ is found to enhance the adsorption of oxygen in the presence of water through changes in near-surface electronic structure; the same effect is not observed on the insulating surface of ThO₂.     

Electrical conductivity measurement and thermogravimetric study of pure and niobium-doped uranium dioxide

The electrical conductivity and nonstoichiometric composition of UO_2+x and (U_1?yNb_y)O_2+x (y = 0.01, 0.05 and 0.10) were measured in the range $\text{1282 ≦ T ≦ 1373}$ K and Pa by tie four inserted wires method and thermogravimetry, respectively. The electrical conductivity of (U_1?yNb_y)O_2+x plotted against the oxygen partial pressure indicated a minimum corresponding to the transition between $\text{n}$- and $\text{p-}\text{type}$ cone uction. The band-gap energy of (U_1?yNb_y)O_2+x was calculated to be $\text{(248 ± 12)}$ kJmol.^?1, independent of niobium content, which is nearly the same as that of UO_2+x. From the oxygen partial pressure dependences of both the electrical conductivity and the deviation $\text{x}$ of UO_2+x and (U_1?yNb_y)O_2+x, the defect structures in these oxides were discussed with the complex defect model consisting of oxygen vacancies and two kinds of interstitial oxygens.     

Electrical conductivity of UO2-ThO2 solid solutions

The electrical conductivities of UO_2+x. ThO₂ and their solid solutions, in thermodynamic equilibrium with the gas phase, were measured as a function of temperature, and of oxygen partial pressure in the temperatnre range 800 to 1200°C. The slope of the plot log α versus 1/$\text{T}$ for UO_2+x and UO₂-rich solid solutions exhibits a single region, whereas in the ThO₂-rich solid solutions it exhibits two regions. The pressure dependence of the conductivity (σ) in the UO₂-rich solid solutions can be represented by σ ∝ [O_i] ∝ in the range of $\text{0.01 < x < 0.1}$. Here, O_i is an interstitial oxygen and the partial pressure of oxygen, and it varies with the ThO₂ content. At greater deviation from stoichiometry ($\text{x ? 0.1}$) the presence of U₄O₉ or (Th U)₄O₉ phases influences the conductivity data. In ThO$\text{2}$ or ThO₂-rich solid solutions. P-type conduction at high oxygen pressures is interpreted as arising from the incorporation of excess oxygen into oxygen vacancies.     

The effect of additives on the irradiation behaviour of UO2

Samples of UO₂ doped with small amounts of Nb₂O₅ or La₂O₃, and having various grain sizes, have been irradiated at 1500°C to 0.1% FIMA. At this low burn-up, gas release and swelling measurements show no dependence on dopant, but the Booth model prediction of swelling proportional to reciprocal grain size has been verified. Gas release does not fit the simple Booth model at the low releases measured, and shows a dependence on sample density, and hence surface area only. A model has been derived to explain these results. The rare gas diffusion coefficient in UO₂ at 1500°C has been measured to be $\text{1.6 × 10}{}^{\mathrm{?19}}$ m²/s.     

Effect of the heat-treatment temperature of powders on oxide fuel quality

It has been determined at the State Scientific Center of the Russian Federation—Physics and Power Engineering Institute in the course of developing a technology for fabricating various fuel compositions (UO₂+MgO, UO₂+ThO₂, UO₂+Th+ThO₂, PuO₂+MgO, UO₂+Fe+MgO, PuO₂+BaO, and others) for fast-neutron and light-water reactors that structural changes in particle aggolmerates occur at the heat-treatment stage. The optimal properties of the powders are obtained at the temperature of the morphological transformations of the particles. The fuel pellets prepared from these powders possess stable density, porosity, exterior form, mechanical strength, and so on. The total specific surface area of the oxides is an indirect parameter for estimating their quality. Each fuel composition has its own optimal powder heat-temperature temperature. 7 figures, 1 table, 5 references. State Scientific Center of the Russian Federation—A. I. Leipunskii Physics and Power-Engineering Institute. Translated from Atomnaya énergiya, Vol. 86, No. 3, pp. 189–194, March, 1999.     

Optical Trappings of ThO2 and UO2 Particles Using Radiation Pressure of a Visible Laser Light

Optical trappings of ThO₂ and UO₂ particles have been first demonstrated in water using the radiation pressure of a TEM₀₀-mode He-Ne laser beam of λ=633 nm. It was observed that a ThO₂ particle was successfully trapped three-dimensionally in the focus region and transferred by moving the focus. On the other hand, for a UO₂ particle of which a refractive index and an extinction coefficient are relatively large in the visible region, only two-dimensional trapping was observed when the beam focus was located near the bottom of the particle. One of the main difficulties in the optical trapping of nuclear fuel particles is attributed to their relatively large absorption coefficients in the visible region. Computational studies on three-dimensional optical trapping performances of absorbing particles were, therefore, perfomed with a simulation code based on geometrical optics. The present calculation can well predict the experimental results on the optical trapping characteristics for ThO₂ and UO₂ particles.     

Thermal Conductivity of ThO2-UO2 System

The thermal diffusivity of the ThO₂-UO₂ system in solid solution was measured by laser pulse method at temperatures ranging from 20° to 500°C. Reproducibility of the data was confirmed to be within 3%. The compositions of the samples were ThO₂, ThO₂-1%UO₂, ThO₂-5%UO₂ and ThO₂-10%UO₂.

The thermal conductivity was calculated from measured thermal diffusivity data and the specific heat data available in literature and corrected to zero porosity by using Loeb's equation, in which the shape factor is unity.

The values on ThO₂ thus obtained agreed very well with the data found in literature, throughout the range of temperature of the experiments. The thermal conductivity of ThO₂, ThO₂-1%UO₂, ThO₂-5%UO₂ and ThO₂-10%UO₂, at 20°C, were 0.0312, 0.0288, 0.247 and 0.0184 cal·cm/sec· °C·cm², respectively.     

Study on Redox Equilibrium of UO2 2+/U2 + in Molten NaCl-2CsCl by UV-Vis Spectrophotometry

In order to enhance the understanding of the redox equilibriums of uranyl ions in molten NaCl-2CsCl eutectic salt UV-Vis absorption spectrophotometry measurements were performed for UO₂ ²⁺ in molten NaCl-2CsCl at 923 K under simultaneous electrolytic control of their ratio. A prominent absorption band at 395 nm was assigned to UO₂ ⁺, and its molar absorptivity was determined to be 832±27 mol^-1·l·cm^-1. From the dependence of the rest potential of the melt on the spectrophotometrically determined ratio of [UO₂ ²⁺]/[UO₂ ⁺], the standard redox potential of the couple UO₂ ²⁺/UO₂ ⁺ was determined to be ?0.903±0.007 V vs. Cl₂/Cl^—.     

Characterization of (Th,U)O2 pellets made by advanced CAP process

Coated Agglomerate Pelletization (CAP) process is being developed by Bhabha Atomic Research Centre (BARC) for the fabrication of ThO₂-UO₂ mixed oxide fuel pellets. In order to improve the microstructures with better microhomogeneity, a study was made to modify the CAP process. The advanced CAP (A-CAP) process is similar to the CAP process except that the co-precipitated powder of mixed oxide, ThO₂-30%UO₂ or ThO₂-50%UO₂, is used for coating instead of U₃O₈ powder. The choice of ThO₂-UO₂ powders as the coating material is advantageous compared to U₃O₈, since the presence of large quantities of ThO₂ in UO₂ powder gives better self-shielding effect. In this paper, ThO₂ containing 4%UO₂ (% in weight) was prepared by the A-CAP process. Property measurements including microstructure and microhomogeneity were made by optical microscopy, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), etc. It was found that the pellets sintered in air at 1400 °C showed a duplex grain structure and those sintered in Ar-8%H₂ at 1650 °C showed a very uniform grain structure with excellent microhomogeneity.     

Densification behaviour of ThO2–PuO2 pellets with varying PuO2 content using dilatometry

The shrinkage behaviour of ThO₂, ThO₂–30%PuO₂, ThO₂–50%PuO₂ and ThO₂–75%PuO₂ pellets has been studied using a dilatometer in inert (Ar) and reducing atmospheres (Ar–8%H₂). The effects of dopants of CaO and Nb₂O₅ on shrinkage of the oxides of the above Pu/(Pu+Th) ratios were also studied. Out of the two dopants studied, CaO was found to give larger shrinkage for all the Pu/(Pu+Th) ratios covered in this study. It was also found that the shrinkage was marginally larger in Ar–8%H₂ than in Ar atmosphere. Addition of PuO₂ to ThO₂ enhanced sintering. This was found to be true for both the dopants. During the sintering of ThO₂, a prominent peak was observed in the shrinkage curve at around 100–300 °C. This peak was attributed to the pressure increase of the trapped gases which subsequently release at high temperatures.     

Densification behaviour and sintering kinetics of ThO2-4%UO2 pellet

ThO₂-?4% ²³³UO₂ fuel will be the driver fuel for the forthcoming Advanced Heavy Water Reactor (AHWR) in India. Densification behaviour such as shrinkage and shrinkage rates of the green pellets of ThO₂-4wt.% UO₂ (natural ‘U’) fabricated by Coated Agglomerate Pelletization (CAP) process were studied using a vertical dilatometer at different heating rates. Activation energy of sintering, ‘Q’, was estimated in the initial stages of sintering by continuous rate of heating (CRH) technique as proposed by ‘Wang and Rishi Raj’ and ‘Young and Cutler’. The sintering mechanism was identified to be as the grain boundary diffusion (GBD) and the average ‘Q’ value obtained by these two methods were found to be 350 ± 16 kJ/mole and 358 ± 5 kJ/mole, respectively.